This paper codifies psychodesign as a clinical–cultural design discipline, treating the built environment as a measurable, adjustable interface between human psychology, culture, and health outcomes. Unlike generic "wellbeing design" or neuroarchitecture alone, psychodesign is proposed to address a practical gap: designing spaces that measurably reduce psychological load and strengthen social functioning in populations facing chronic stressors, historical trauma, or cultural dissonance. The discipline integrates three non-negotiable gates: (1) explicit psychological hypotheses and mechanisms (Clinical gate), (2) locally valid cultural-symbolic constraints (Cultural gate), and (3) a reproducible evaluation protocol with measurable outcomes (Measurement gate). The core contribution is the introduction of a minimal vocabulary and the formalization of the approach through a causal chain model (Inputs (Design variables) → Mechanisms (psychological mediators) → Outcomes (measured)). It presents a four-category Intervention Taxonomy (Stress-regulation, Identity and Meaning, Social Cohesion, Institutional Repair) and a detailed Measurement Specification suitable for resource-constrained contexts, including the Cultural Alignment Score (CAS). Finally, it outlines a four-step pilot protocol and establishes robust Governance and Ethics requirements, particularly the need for community audit rights and anti-capture controls, explicitly forbidding tokenistic “participation theater.” This framework aims to establish psychodesign as a falsifiable, reproducible field discipline focused on real-world psychological improvement.